Cathode ray tube



Patented Apr. 16, 1940 p I v 2,197,523v

UNlTED STATES PATENT OFFlCE OATHODE RAY TUBE Dennis Gabor, Rugby, England, asslgnor to General Electric Company, a corporation of New York Application October 13, 1937, Serial No. 168,820 In Great Britain October 30, 1936 7 Claims. (Cl. 250-158) This invention relates to improvements in elecnew gun and deflector system, and Fig. 16 is a tl'ostatic deflector systems of cathode-ray oscildiagrammatic representation showing the manlographs and television tubes. Its object is to her of connection and energization of the deprovide a new deflector system, with high deflecting electrodes.

.y'flection sensitivitywhich is suitable for much In Fig. 1, which is a longitudinal section of a Wider deflection angles than hitherto used, and particularly simple design of a deflector system allows constructing very short cathode-ray tubes according to the invention, 1, 2, 3 and d are dewith large screens. flector plates, Which are connected crosswise, i. e.,

One feature of the invention comprises an im l is connected with 4 and 2 with 3. The two sysv 7 proved deflector system which includes two sets terns 2 and3-4 will therefore deflect the elecof successively arranged deflecting electrodes, trons in opposite directions. Systems of crosssuch electrodes being so energized as to produce wise connected deflector plates have been already opposite oscillatory deflections of the electron used in cathode-ray oscillographs in so-called stream. A further feature of the invention conbeam traps, i. e., in designs in which it was .5 sists in arranging the various sets of deflecting intended to cut off the undeflected beam, or, in electrodes in the configuration of split cylinders some systems, to cut off the deflected beam. It and by this. means producing deflections in two was not known, however, that such systems if planes at a common pivot point. According to a suitably dimensioned can have deflecting sensifurther feature of the invention, the deflecting tivities superior to simple deflecting condensers.

:0 system is followed by an electrostatic lens hav- This seems surprising, as the two systems are acting its first nodal point coincident with the pivot ing against each other. The explanation is conpoint of the deflecting system, or as near to such tained in Fig. 1, which shows the case of optimum point as possible. According to a still further dimensions, in which the length of the first defeature of the invention, in order to eliminate the fle tor y tem is of the length 33 of the secastigmatism of h deflecting y h fl in and system, i. e., of the total length. In this 25 syst is s rr d as t produce a ssv case of the length of the second systemis used of the ys of the electron b at a point Withfor compensating the deflection angle produced in the deflecting structure. As a further aspect by th first system, Inthe remaining astrongof this feature, an electrostatic lens follows the r deflection i the opposite direction is produced.

deflecting syst in such, location that the lens The deflected beams describe approximately par- 30 focuses an ima of t sa cross-over on the abolic paths. It follows therefrom that the defluorescent screen. The pivot point of the beam flected beam 111 pivot around point :v t is caused to coincide as nearly as Possible With distance from the hind edge of the plates, equal t e nodal Point Of the lensto of the total length of the system. Thereby In the drawings, Fig. 1 is a Sketch of the new very large deflecting angles are produced. At 35 d fle in system in h simplest s Fi 2 the maximum deflecting voltage at which the and 3 arc-drawings O a d fl c System Which beam can still pass through the plates it just produces deflections in two perpendicular direcgrazes the plate 3 and the edge of 4. We see, t C' i illustrates a Complete electron g therefore, that the spacing of the plates is fully fittedwith such a defle y m; 5 is & utilized in this system, whereas it is only half deflector System With a lens behmd 113; 6 utilized in simple deflecting condensers. Calling is a diagram showing the cardinal points of said E the driving Voltage f the electrons v the lens and the refraction of electron rays in it; maximum d fl ti Voltage and a the maximum Flgs- 7 to dl'awmgs of mPdlfied deflector deflecting angle, the following relation holds for l5ws7stemx i 10 1S a'skebch -W the t f simple condensers: V/E=2tan a whereas for the resultmg i e1ectrst.at1c 9' n fl system shown in Fig. 1 this becomes V/E=tall oz.

11 shows the principle by which this is ehmlnat- Th d n ti n n it m in m f 1 ed in accordance with the present inventionyFig. 8 ac o S W y 9 e. or g 12 is an example of an anastigmafic gun and maximum deflect ng angle 15 therefore doubled in the new system. 0

3O .flector system according to the invention; Fig. 13 5 is a diagram explaining the refraction of elec- E de5c11bef1 c a$e1 m whlch 32/ $1 is 3, glVeS tron rays in Fig 1 Fig" 1 1 an improved maximum sensitivity. If 1:2 is chosen larger, the

sign of an electron gun according to the invendeflected beam Will be @1117 Off y the edge of 4, tion; Fig. 15 is a drawing of a complete cathodeif an is chosen smaller, an advantage will be ray oscillograph or television tube fitted with the retained as compared with the simple system so long as the edge of the second system is behind the point Q, at which in the optimum system the deflected beam cuts the axis. In this case m2/x1=2.41l, and the pivot point coincides with the ends of the deflector system. In these approximate calculations the edge field has not been taken into account. In reality the optimum ratio 332/.1'1 is smaller than 3, as theedge fields add to the deflection produced by the second system, but detract from the deflections produced by the first.

The new system offers particular advantages, if each of the successively arranged two deflecting systems is shaped as a split ,tube. This is shown in Figs. 2 and 3 which are side and end views respectively of such a system. The cylinder segments 5 through 8 form the first, and 9, it!

and ll'with its opposite segment, not shown in the drawings, form the second system. 8 is connected with 9, 5 and Ill, etc. If the condenser systems 58 and 'J8 are connected with potentials symmetrical to the same potentiallevel, the field between them is'approximately homogeneous over a great part of the cross section. This system will therefore produce deflections in both directions and occupy only the space of a single deflector system.

Fig. 4 shows a complete electron gun and deflector system of the last described kind. In

Fig. 4, E2 is the cathode, It the modulating grid,

i l the first anode, E6 the second anode, II the first and E8 the second deflector system. The mean potential of the deflector systems is equal to the final driving voltage .of the tube. Therefore 5 between it and El, an electron lens formed,

which projects the image of the, cathode, or of the cross-over formed near the opening of it on the screen. slender shape of this system, very wide deflecting angles are obtained. According to the approximate calculation with parabolas the maximum angle in this case is 36, in reality it can be.

ev n la e as the edge field of l 8 increases the deflection after the. electronshave left the system.

According to a further feature of the. invention, the deflecting sensitivity of this system can be even further increased by the method known per se of deflecting the electrons before they have obtained their final velocity. As shown in Fig. 5, this can be effected in a particularly simple Way, by omitting the second anode l 6 in Fig. 4 and using the deflector systems themselves as second anode. The final lens can be obtained by joining a further ring to the system or, as shown in Fig. 5, by using the conducting coating of the tube itself as final anode. In this figure I9 is the cathode, 28 the grid, M the first anode, 22 the first deflecting system, 23 the second deflecting system, and 24- the conducting coating on the tube envelope. It is of course understood that the electron gun and the lenses as shown in this and the following examples are chosen also by way of illustration and can be replaced by any other known guns or lenses.

Fig. 6 is a diagram, by means of which the resulting deflecting sensitivity can be obtained. This diagram shows the cardinal points of the electron lens corresponding to the actual dimen sions of Fig. 5, in the case where the mean potential of the deflecting system is A of the final voltage, i. e., if the total driving voltage is 4000' volts and the mean potential of the deflecting plates F1 is the first, F2 the second focusis 1000 volts. of the electron lens so formed, H1 and H2 are its It is seen that in spite of the two principal planes. By the construction as shown, we can, according to the rules of geometrical optics as well known in connection with thick lenses, obtain the resulting deflection under the approximate assumption, that deflection and concentration are superposed, but do-not interfere with each other. We see that the initial deflected beam I leaves the system as the ray II, coming from another pivoting point, under an angle which is only about 58% of the original angle. As a result, the deflecting sensitivity of the system is reduced below the value which would obtain it the electrostatic lens efiect were not present.

s This can be avoided according to the invention, by passing the deflected beam through the first nodal point of the lens. As is shown in the optical theory of thick lenses, every lens has two nodal points of the following properties: A ray passing from the'object space through the first nodal point N1 after having passed through the lens. will appear coming from the second nodal point Nzf in a direction parallel to the original ray. These nodal points can be constructed as shown in Fig. 6. The distance of N1 from F2 is equal to ii, the first focal length, and the distance of N2 from F1 is equal to the second focal length f2. It is seen that in Fig. 5 the nodal point N1 is at far too great a distance from the pivot point P.

Far higher deflecting sensitivities can be obtained according to the invention by the design as shown in'Figs. 7 and 8. Here 25 and 26 are the two deflecting systems. The second is fitted with a conical part 2'! and the pivot point P of the rays lies well outside the opening. By the conical shape, however, considerably higher sensitivities are produced; the sensitivity of the design shown in Fig. 7 is for the same deflecting angle very nearly the same as of the design in Fig. 1. Moreover, deflecting fields and focusing fields are more effectively separated. This is even better achieved in the design shown in Fig. 9, in which the deflecting system is covered from the screen side by a truncated'cone 28. By giving this cone suitable potentials, somewhat negative against the mean potential of the deflecting system, the penetration of the outer field can be reduced to a great extent. The potential of 28 must not be made too negative, as in this case a lens is formed within the entrance. By this design it is possible to make the deflected beam pass exactly through the first nodal point of the lens.

According to a further feature of the invention, the new deflector system can be operated with suitable combinations of lenses in such a way as to eliminate the astigmatism which up to now has been a major disturbing factor in cathode-ray tubes, especially in tubes for television purposes.

Fig. explains-the phenomenon of deflection astigmatism. 2S and 30 are the two plates of a deflecting condenser. a and b are two electron trajectories, which are parallel if no deflecting field is acting. If, however, a field is produced, the deflected rays will cross over, as the ray a nearer the negative plate 25- will suffer stronger deflection than the ray 1). This has two causes: Firstly, the deflected ray b travels a shorter distance in the deflecting field than the deflected ray a and secondly, as it is constantly at a higher potential, it is stiffer than a. In magnetic deflecting fields the astigmatism is generally much smaller, as there the second factor rules out.

Mathematically the astigmatism in an electrostatic deflecting field can be expressed by the following relation:

at 2 Apia??? ln ds 1) In this formula Act is the diiference between the angles by which two rays have been deflected, which have passed through a deflecting field, at a distance AN from each other; the distance varying in general along the path S.\,l/ is the potential, measured against the cathode. tip/ and tap/5N are the first and the second partial derivatives with respect to the direction 11, perpendiculan to the trajectory. Ifas in most CELSESA:1 is small compared with a, we can calculate this integral with sufiicient accuracy assuming that AN has varied in the same way along the path s as if no deflection had taken place. In the case shown in Fig. we could e. gassume AN:AN1=constant. We can then carry out the integration if we only know cap/6N and 5 tb/5N i. e., the potential, the gradient normal to the path, and the gradient of the gradient normal to the path, for one deflected trajectory. In first approximation we can even substitute the values for the undeflected trajectories.

In the case of electrons of the volt-energy E in a homogeneous deflecting field with the constant gradient F, in which the first term in the integral vanishes, this becomes simply 3 F 2 Act EA dx (2) In view of the small difierence between the arc length a and the ordinate parallel to the system axis x I have substituted the latter. Vv'e see from this how a homogeneous deflecting field produces astigmatism. The non-homogeneous fring ng field of the deflecting plates, which is widely be- I lieved to be responsible for it, is in reality beneficial, but its effect is insuificient to compensate it.

According to the invention, I eliminate astigmatism by providing inside the deflecting system a crossing point of the electron rays, i. e., an

L image of the cathode or of the cross-over. The above expression (2) then becomes equal to zero, as An assumes now positive and negative values.

It can be shown, however, that this method is l of no avail in the case of ordinary deflecting systems, but is applicable only to systems according to the present invention, in which deflections are effected in opposite directions. Fig. 11 shows a parallel condenser system, in which an image is i produced at the center C. According to equation 2 this system is anastigmatic. W e can not utilize it, however, for the following reason: If we want to obtain eventually an image or C on the screen, a lens must be placed behind the deflecting systern. As, however, the screen. is at a large distance, this means that C must nearly coincide with the first focal point of the lens. As now the pivot point in this deflecting system is also C, this means that the deflection will be nearly 3 completely destroyed by the lens.

This could be assisted by using wedge-shaped or curved deflecting plates. It turns out, however, that the anastigmatic center, i. e., the point in which the rays must cross in order to make 1 the astigmatism zero, will always follow the pivot point at a close distance.

This is, however, quite different in the systems according to the invention, as shown in Fig. 12. Here 3! is the cathode, 32 the grid, 33 the first 5 anode and 34 the second anode. 35 and 36 are the two deflecting systems, consisting of split cylinders. The anastigmatic center C is in this case again near the center of the whole deflecting system, whereas the pivot point P is at its end, inside the final lens, which is formed between 36 and the final anode 37. It is therefore possible to focus C on the screen without destroying the .deflections. The intermediate image C is produced in this example simply by the lens formed between 3 3 and 35, if the mean potential of 35 is different from the potential of. 3 1. It can be higher or lower; in any case the lens will be a concentrating lens.

The diagram in Fig. 13 shows that the deflection is not destroyed but even increased. this arrangement. This diagram corresponds to the case when the voltage of 3'! is seven times the mean potential of the deflecting systems. This potential ratio is necessary for focusing C on the screen at a distance of about 30 cms. The construction shows that the deflection angle is reduced to about 521%. This means that the sensitivity is still 7x0.52 =l.9 times greater than obtainable with a parallel condenser system and the astigmatism is eliminated.

The most advantageous design is, however, the one shown in Fig. 14, which shows a combination of the same electron gun system as in Fig. 12 with the new deflection system as shown in Fig. 9. With this system it is possible to fulfill both conditions which according to the invention make the deflection sensitivity large and the astigmatism zero, namely: The focal point of the final lens nearly coinciding with the anastigmatic center of the deflecting system, and the pivot point of. the deflecting system nearly coinciding with the first nodal point of the lens.

With this system it is possible to obtain large deflections without astigmatism, and therefore it is possible to make the cathode-ray tube short and the aperture large. This leads to a television tube construction as shown in Fig. which at equal screen size has a length only of /g-V; of the tubes used at present. Apart from the obvious advantage following from the short length, this oscillograph allows obtaining much smaller spots and much higher brilliance, because both the short length and the large lens aperture are helping to reduce the spot size. Because of the high deflecting sensitivity it requires also only small scanning voltages, and by this helps to reduce the space occupied by the scanning devices in television sets.

In Fig. 16, the cross-connection system referred to in connection with Figs. 1 to 4 is shown in detail. In the arrangement illustrated, electrode 5 is connected to electrode H] by means of a conductor 40, electrode 5 to electrode 9 by means of a conductor 4|, electrode 7 to electrode l2 by means of a conductor t2, and electrode 8 to electrode H by means of a conductor 13.

In order to energize the deflecting system, a common source 45 of alternating scanning potential is connected across the conductors 5D and ll. In view of the mode of connection indicated it is apparent that the potential applied to the electrode pair 5, B, is identical in periodicity but opposite in direction of deflecting action to that applied to the electrode pair 9, l0. Consequently, these pairs produce opposite oscillatory deflections of the electron beam traversing them. Similar functioning of the electrode pairs 1, 8 and II, l2 may be obtained by applying to them a second scanning potential from an appropriate source (not shown).

In this application only the outlines of the invention have been described and a few examples given. Other applications of the new principles involved-e. g. for television tubes with magnetic focusing and scanning, separately or togetherwil1 be obvious to those skilled in the art.

What I claim as new and desire to secure by Letters Patent in the United States is:

1. In combination, an electron gun for projecting an electron beam along a given axis, a screen to be scanned by the beam and a deflecting system efiective to cause t beam to scan the screen, the said system including a frst electrode structure operative to produce initial cscil latory deflection of the beam. and a second electrode structure arranged sequentially to the first for producing additional oscillator defiection of the beam in a direction opposite to as said initial deflection, the resulting pivoting point of the beam being adjacent the end of the said second electrode structure which is nearest to the screen, and an apertured electrode positioned between the said second electrode structure and the screen forming therewith an electron-optical lens, the second electrode structure being tapered the screen end and the apertured electrode being shaped to conform to the said taper, whereby the nodal point of the said electron lens is caused to coincide at least approximately with the said pivoting point of the electron beam.

2. In combination an electron gun for projecting an electron beam along given axis, a screen to be scanned by the beam and a deflecting system effective to cause the beam to scan the screen, the said system including a first electrode structure operating to produce initial oscillatory deflection of the beam and a second electrode structure arranged sequentially to the first for producing additional oscillatory deflection of the beam in a direction opposite to the said initial deflection, the beam being thus given an effective pivoting point relatively near to the l screen end of the said second electrode structure,

means including an electron-optical lens adjacent the gun end of the electrode structure for producing a cross-over point within the said second deflecting electrode structure at a point different from the said pivotal point and a second electron-optical lens adjacent the screen end of the deflecting structure, the said lens having its focal point substantially coincident with the said cross-over point.

3. A cathode ray tube comprising means including a cathode for producing a beam of electrons, a screen to be scanned by the said beam, beam deflecting structures arranged at two spaced positions along the axis of the tube, said structures including a pair of oppositely. disposed electrodes at each position, each of the electrodes at one position being connected respectively to its oppositely disposed electrode at the other position whereby the beam is deflected in opposite directions in passing successively through said structures, means including an electron-optical lens adjacent the cathode end of the first deflecting structure for producing a crossover point Within the said second deflecting structure and a second electron-optical lens adjacent the screen end of the second deflecting structure, the said second lens having its focal point substantially coincident with the said cross-over point whereby anastigmatic deflection of the beam is obtained.

4. A cathode ray tube comprising means ineluding a cathode for producing a beam of electrons, a screen to be scanned by the beam, two beam deflecting structures arranged sequentially along the axis of the tube, each structure including two pair of mutually perpendicular electrodes arranged in substantially closed configuration and each of the electrodes of the first structure being connected to an oppositely disposed electrode of the second structure, whereby the beam is deflected about a pivoting point relatively close to the screen end of the second structure, means including an electron-optical lens adjacent the cathode end of the first beam deflecting structure for producing a cross-over point within the second deflecting structure and means including a tapered portion at the screen end of the second deflecting structure and a conical electrode conforming with the said tapered portion for providing an electron-optical lens having its focal point substantially coincident with the said crossover point, whereby the deflection of the beam is rendered anastigmatic.

5. A cathode ray tube comprising means for producing a cathode ray beam, a screen to be scanned by the beam and a deflecting system effective to cause the beam to scan the screen, the said system including a first electrode structure operative to produce initial oscillatory deflection of the beam and a second electrode structure arranged sequentially to the first for producing additional oscillatory deflection of the beam in a direction opposite to the said initial deflection, the resulting pivoting point of the beam being adjacent the end of the said second electrode structure which is nearer to the screen, and an apertured electrode arranged in. partially telescoping relation with the said end of the second electrode structure, the two last-named parts being of such configuration as conjointly to provide an electron lens having a nodal point approximately coincident with the said pivoting point of the beam.

6. In combination, means for projecting an electron beam along a given axis, a fluorescent screen to be scanned by the beam, and deflecting means including two mutually perpendicular electrode systems operable to cause the beam to scan the entire surface of the screen in successive increments, each of said electrode systems including a first pair of electrodes efiective when subjected to a potential difference to exert a deflecting force on the beam in a given plane parallel to the beam axis, a second pair of electrodes arranged sequentially to the first and in substantial alignment therewith, said second set of electrodes being dimensioned to act on the beam for a substantially greater portion of its path length than the first pair and effective when subjected to a potential difierence to exert a deflecting force in the same plane as said first pair, and means for applying to said first pair an alternating potential difference of constant amplitude and desired scanning periodicity and to said second pair an alternating potential of sirnilar amplitude and periodicity but of reversed direction of action.

'7. In combination, means for projecting an electron beam along a given axis, a fluorescent screen to be scanned by the beam, and deflecting means including two mutually perpendicular electrode systems effective to cause the beam to scan the entire surface of the screen in successive increments, each of said systems including a first pair of oppositely disposed electrodes Opond pair and there being a common source of alternating potential of constant amplitude for energizing both pairs of electrodes whereby the respective electrode pairs are effective to produce opposite oscillatory deflections of the electron 5 beam.

DENNIS GABOR.

Certificate of Correction Patent No. 2,197,523. April 16, 1940. DENNIS GABOR It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows: Page 2, first'column, line 13, for the Words successively arranged two read two successively arranged; page 3, first column, lines 3 to 5 inclusive, strike out the formula and insert instead the following l & L I A0L262 N 5N A ds line 22, for fill/5 read 5 0/6 and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Oifice.

Signed and sealed this 11th day of June, A. D. 1940.

[SEAL] HENRY VAN ARSDALE,

Acting Commissioner of Patents, 

